Phase-locked loop circuit as well as a voltage-controlled oscillator as used in a phase-locked loop circuit

ABSTRACT

A phase-locked loop circuit comprises a voltage-controlled oscillator with at least one resonator circuit for driving the oscillator to an output frequency which is a multiple of the resonator frequency. The voltage-controlled oscillator is connected to a phase-locked loop comprising frequency control means for controlling the output frequency of the oscillator. According to the invention the resonator circuit comprises at least one adjustable component and the frequency control means are coupled into the resonator circuit for controlling the resonator frequency of the resonator circuit. The invention moreover relates to a voltage-controlled oscillator as used in this phase-locked loop circuit.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority of European Patent Application No.00308559.4, which was filed on Sep. 29, 2000.

BACKGROUND OF THE INVENTION

The advent of wireless communications at C-band frequencies has fueled ademand for low cost high performance integrated circuits. GH_(z) radiofront ends are designed in order to meet the needs of the thirdgeneration wireless access systems in terms of bit rate andimplementation constraints (size, power consumption, and cost). For thispurpose, fully integrated transceivers are a common interest nowadays.Such a transceiver chip is generally coupled to a local oscillator (LO)which provides a clock for data transmission and reception. Generally,the oscillator is thereby connected in a phase-locked loop (PLL) circuithaving a phase-locked loop with frequency control means to confine theoutput phase and frequency of the oscillator within acceptableboundaries.

Often the oscillator is formed out of a number of distributed microwavecomponents, for instance micro strip lines, which are designed toacquire the desired electronic behaviour and characteristics. Dependingon their shape and dimensions such distributed components may act eitheras an inductor, capacitor, resistor or conductor for the supplied signaland they are formed to render the appropriate function within thecircuit. These components, however, are difficult to be integratedtogether with a radio-frequency (RF) system or a PLL-circuit, due totheir dimension at microwave frequency. Hence, these components willusually require additional packages and external interconnections.Moreover, these distributed components are generally poorly tunable, astheir electrical characteristics strongly depend on the signalfrequency.

European patent application 689.287 discloses the basis setup of a moreintegrated approach of a phase-locked loop circuit with avoltage-controlled oscillator connected to a phase-locked loop. ThePLL-circuit comprises frequency control means in the form of a phasedetector which is coupled to the output of the voltage-controlledoscillator, while being fed by a reference signal. The differentialsignal of the phase detector is fed back to the voltage-controlledoscillator via a charge pump and a loop filter to correct any deviationfrom the intended output frequency. A voltage-controlled oscillator, asused in this prior art circuit, has the advantage that the outputfrequency can be adjusted to certain extent by means of the voltagesupplied to the oscillator, which renders the device suitable fordifferent operating speeds according to different industrial standards.

Although voltage-controlled oscillator can be designed to operate atvery high speed, the phase-locked loops which are commercially availablemostly cannot handle the high oscillator frequencies required in thenext generation wireless communication systems of well beyond 20 GH_(z).A known solution to this problem is down converting the fundamentaloscillator frequency to a level the phase-locked loop can cope with.Another approach is to operate the voltage-controlled oscillator at alever suitable for the phase-locked loop and up-converting theoscillator frequency to a level required by the RF system according toindustrial standards. Both solutions, however, require additionalcircuit complexity, more packages and more circuit components, e.g. afrequency divider or frequency multiplier, and hence extra chip area,more power consumption together with vulnerable interconnections.Moreover more design uncertainty and frequency noise will be introducedby these additional circuits.

It is an object of the present invention to provide a phase-locked loopand a voltage-controlled oscillator of the kind referred to in theopening paragraph which meet the above drawbacks at least to asignificant extent.

SUMMARY OF THE INVENTION

To this end, the present invention relates to a phase-locked loopcircuit comprising a voltage-controlled oscillator which comprises atleast one resonator circuit for driving the oscillator and aphase-locked loop comprising frequency control means for controlling theoutput frequency of said oscillator, whereby during operation saidresonator circuit runs at a resonator frequency to drive said oscillatorat an oscillator output frequency which is an even integer multiple ofthe resonator frequency. The invention moreover relates to avoltage-controlled oscillator as used in this phase-locked loop circuit.

The above-described phase-locked loop is characterized in that theresonator frequency is coupled to the frequency control means of thephase-locked loop, in that the resonator circuit comprises at least oneadjustable component to control the resonator frequency and in that thephase-locked loop frequency control means are coupled to the resonatorcircuit for controlling the resonator frequency. A RF frequency neededby a RF system may be generated at one or more fundamental virtualground nodes of the oscillator circuit and coupled with or without ahigh-pass filter to the RF system. The resonator circuit, however, runsat a frequency which is only an integer division (1/N, N=2n, n=1.2.3 . .. ) of said RF frequency generated by the oscillator. The reducedfrequency resonator signal may be derived from one or more fundamentalnon-virtual ground nodes in the oscillator to be fed to the phase-lockedloop. Accordingly, the phase-locked loop used in the circuit accordingto the invention only need to handle 1/N the RF-frequency while itcontrols the full output RF frequency of the oscillator. No additionalfrequency divider or multiplier is hence necessary to boost theapplicability of a standard phase-locked loop to a higher frequencylevel than it can handle directly.

A specific embodiment of the phase-locked-loop circuit according to theinvention is characterized in that the voltage-controlled oscillatorcomprises a push-push circuit whose operating frequency is determined bya pair of resonator circuits and in that the frequency control means arecoupled into at least one of said pair of resonator circuits. Apush-push circuit is a balanced circuit in which two active devicesoscillate at the fundamental frequency 180° out of phase and in whicheven harmonic signals are generated in phase. In a proper operation ofsuch a push-push circuit, power is coupled to a load only at evenharmonic frequencies, while the fundamental signals cancel. Accordingly,the oscillator output signal frequency is an even multiple of that ofthe resonator circuits. A single-ended even-harmonic output signal ofthe oscillator may be fed into a RF system, such as a transceiver, whilea differential fundamental output signal of the resonator circuits maybe supplied to the phase-locked loop for frequency control, inaccordance with the present invention, to stabilize the full oscillatoroutput frequency.

In a more particular embodiment, the phase-locked loop circuit accordingto the invention is characterized in that the push-push circuitcomprises a pair of substantially identical active devices being coupledto said resonator circuits, and in that an output signal which is fedback to the resonator circuits to control said active devices.

A preferred embodiment of the phase-locked loop circuit according to theinvention is characterized in that the active devices are transistors,more particularly field effect transistors, and in that any othercircuit components of said push-push circuit and said resonator circuitsare designed to fit in the manufacturing process of said transistors.These other components generally merely consist of passive devices suchas capacitors, inductors and resistors. By designing these components tofit in the same manufacturing process as that of the transistors, theentire circuit may be integrated in a single, common semiconductor body,leading to a substantial cost reduction.

A further embodiment of the phase-locked loop circuit according to theinvention is characterized in that the resonator circuits compriseLC-circuits with at least one variable capacitor or varicap in saidLC-circuits being controlled by the frequency control means. Such aLC-circuit can relatively easily be designed to offer the requiredresonator frequency, which is then doubled (N=2) by the push-pushcircuit. The varicap not only presents a means of varying the circuitsoutput frequency over several frequency bands to suit a particularapplication, but moreover may be coupled to the frequency control meansto correct any deviation from the intended output frequency. A moreparticular embodiment is thereby characterized in that the LC-circuitscomprise at least two variable capacitors each to expand the variationof the output frequency over even a wider range.

For wideband oscillators another requirement is to keep a constant loopgain over the whole frequency range. For this purpose a furtherpreferred embodiment of the phase-locked loop circuit according to theinvention is characterized in that the active devices are each coupledin series with an inductor. Instead of being connected directly to apower supply, the active devices are each connected in series with aninductor. The inherent property of higher Q at higher frequencies ofthese inductors compensates a down slipping-loop gain at the higher endof a frequency band. These inductors are separate from the resonatorcircuits. The requirements of an absolute high Q for these devices ishence not critical.

To counteract frequency noise on the oscillator output signal, a furtherpreferred embodiment of the voltage-controlled oscillator according tothe invention is characterized in that the resonator circuits areconnected to a fixed potential via a load resistor. Said load suppresseseven modes between the pair of resonator circuits as only even mode willpresent a voltage drop over the resistor. Even mode currents may henceeffectively be drained away be said load resistor.

A further preferred embodiment of the voltage-controlled oscillator andphase-locked loop circuit according to the invention is characterized inthat at least the voltage-controlled oscillator merely comprisesintegrated components being integrated in a single semiconductor body.As far as these components are concerned no additional packages andexternal interconnection are required, which considerably savesmanufacturing cost and adds reliability.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in greater detail hereinafter, by way ofexample only, with reference to the accompanying drawing in which:

FIG. 1 shows the basic setup of a prior art phase-locked loop circuitwith a voltage-controlled oscillator;

FIG. 2 shows the basic setup of a specific embodiment of a phase-lockedloop circuit in accordance with the present invention; and

FIG. 3 shows a circuit design of a specific embodiment of avoltage-controlled oscillator in accordance with the invention, as usedin FIG. 2.

DETAILED DESCRIPTION

A basic block diagram for a typical phase-locked loop circuit containinga voltage-controlled oscillator according to the prior art is shown inFIG. 1. An input reference signal with a frequency f_(ref) is comparedby a phase detector inside the phase-locked loop PLL with the outputsignal of the voltage-controlled oscillator VCO having a single outputfrequency f₀. Based upon this comparison, the phase detector produces anoutput signal which is fed to the voltage-controlled oscillator VCO tore-adjust, if necessary, the operating frequency of the oscillator VCOin order to counteract any frequency drift which may occur at the outputof the oscillator VCO during operation.

If this operating frequency f₀ is too high for a commercially availablephase-locked loop, a frequency divider 1/n is connected between theoutput of the voltage-controlled oscillator VCO and the phase-lockedloop PLL, as shown in FIG. 1, in order to reduce the required operatingfrequency of the phase-locked loop PLL by a factor n. Another prior artapproach for the same problem is to generate a low frequency with thevoltage-controlled oscillator which can still be handled directly by thephase-locked loop and to multiply the low oscillator output frequency bya factor n before it is fed to a RF system.

A disadvantage of both these prior art solutions to extent theapplicability of a commercially available phase-locked loop to highspeeds is that a separate unit is required to divide or multiply theoperating frequency f₀ of the oscillator in order to enable an availablephase-locked loop to control the output frequency. This does not onlyadd to the complexity of the total circuit but could even mean theseparate distributed components need to be used which considerably addto the total cost of the circuit.

FIG. 2 gives an example of a basic setup of a phase-locked loop circuitand voltage-controlled oscillator according to the invention whichobviates this drawback, at least to a considerable extent. In this case,an input signal for the phase-locked loop PLL is taken from a resonatorcircuit T within the voltage-controlled oscillator VCO instead of at itsoutput. The phase-locked loop PLL directly controls the operatingfrequency f₀/N of said resonator circuit T which is an even division (N)of the output frequency f₀ of the oscillator. Because the operatingfrequency f₀ of the oscillator is directly determined by the resonatorfrequency f₀/N, the voltage-controlled oscillator is effectivelycontrolled in this manner by a phase-locked loop, which only needs torun at the reduced resonator frequency f₀/N. The voltage-controlledoscillator according to the invention hence inherently generates twofrequencies, a high RF output frequency for a RF system and a lowinternal frequency which is 1/N of the RF output frequency which can beused by the phase-locked loop or any other frequency control means forfrequency control.

A circuit design of a phase-locked loop circuit embodying the inventionis given in FIG. 3. The circuits contains a voltage-controlledoscillator with a push-push topology. This is a balanced circuitcontaining two at least substantially identical and symmetricallyarranged circuit structures, each comprising an active device 1 which,in this example, consists of a field effect transistor (FET). The activedevices 1 are provided with constant current by a current source 14. Theactive transistors 1 are driven by two identical resonator circuits ortanks 13 which operate at fundamental frequency f₀/2. Second harmonicsignals at f₀ are generated in phase at a fundamental virtual groundernode (4), which doubles the frequency range and introduces a highloadable quality factor Q. This improves the load pulling performanceand phase noise.

In stead of coupling a phase-locked loop or any other frequency controlmeans to the output 4 of the oscillator, a signal for these means istaken directly from a differential node pair 15 of both resonatorcircuits 13 at fundamental signal f₀/2, which simplifies the front-endsystem architecture, avoids the need for an additional frequency dividerand reduces current consumption as well as chip size.

For wideband oscillators another requirement is to keep a constant loopgain over the whole frequency range. For this purpose the drain of thetransistors 1 is connected via an inductor 12 instead of directly to thepower supply. The inherent property of higher Q at higher frequencies ofthese inductors 12 compensates a down-slipping loop gain at the higherend of a frequency band. These inductors 12 are apart from the tanks 13,therefore the requirement of their absolute high Q is not critical.

Both tanks 13 comprise a substantially identical LC-circuit thatoperates at the fundamental frequency f₀/2. These LC-circuits comprisean inductor 16 together with a variable capacitor 18. These varicaps 18may not only be used as frequency correction means to which thefundamental output signal may be fed, but moreover enable tuning of thedevice over different frequency bands. In this example, extra capacitivefeedback between the source and gate of the transistors 1 is introducedby using two extra variable capacitors 1 to realize multi-band operationwithout substantially adding to the complexity or cost of the totalcircuit.

Even mode suppression is realized by a load resistor 19, which connectsthe resonator circuits to a fixed potential and provides also a DC feedof the variable capacitors 13. Different to odd modes, even modes willgive rise to a net potential over the load resistor 19 and hence willdrain away. Even mode oscillations at the fundamental frequency areaccordingly effectively cancelled.

All components used in the circuit of FIG. 3 are designed to fit in anexisting GaAs manufacturing process in order to be integrated in one andthe same, common semiconductor body. This greatly reduces the total costof the circuit. Moreover, by using merely lumped components for theoscillator and resonator circuits, the circuit can be applied atdifferent frequency band without substantial performance degradationwhich would occur if micro strip like distributed components were used.

Although the invention has been described with reference to merely theforegoing embodiment, it will be appreciated that the invention is by nomeans limited to the embodiment given. A skilled person, on thecontrary, will be able to arrive at numerous other embodiments andvariations without departing from the scope and spirit of the presentinvention. As such he may avail himself of different kinds of push-pushand resonator circuits or even base the oscillator circuit design on adifferent topology. Instead of using field effect transistors, bipolartransistors might as well be used as active devices in such a structure.In stead of using a phase-locked loop as frequency control means, othermeans could be used to control and correct the output frequency of theoscillator. In line with the present invention these means are also fedwith a signal taken from the resonator circuits which drive theoscillator, such that the frequency control means can be operated at amoderate operating speed being 1/N the output RF frequency of theoscillator, where N is an even integer.

1. Phase-locked loop circuit comprising: a voltage-controlled oscillatorwhich includes at least one resonator circuit for driving theoscillator; a phase-locked loop including frequency control means forcontrolling the output frequency of said oscillator, such that duringoperation said resonator circuit runs at a resonator frequency to drivesaid oscillator at an oscillator output frequency which is an integermultiple greater than one of the resonator frequency; wherein theresonator frequency is coupled to the frequency control means of thephase-locked loop, and wherein the resonator circuit includes at leastone adjustable component to control the resonator frequency. 2.Phase-locked loop circuit according to claim 1, wherein thevoltage-controlled oscillator comprises a push-pull circuit whoseoperating frequency is determined by a pair of resonator circuits and inthat the frequency control means are coupled into at least one of saidpair of resonator circuits.
 3. Phase-locked loop circuit according toclaim 2, wherein the push-pull circuit comprises a pair of substantiallyidentical active devices being coupled to said resonator circuits, andin that the frequency control means comprise a phase detector which iscapable of generating an output signal which is fed back t the resonatorcircuits to control said active devices.
 4. Phase-locked loop circuitaccording to claim 3, wherein the active devices are field effecttransistors.
 5. Phase-locked loop circuit according to claim 3, whereinthe resonator circuits comprise LC-circuits with at least one variablecapacitor in said LC-circuits being controlled by the frequency controlmeans.
 6. Phase-locked loop circuit according to claim 5, wherein theLC-circuits comprise at least two variable capacitors each. 7.Phase-locked loop circuit according to claim 3, wherein the activedevices are each coupled in series with an inductor.
 8. Phase-lockedloop circuit according to claim 2, wherein the resonator circuits areconnected to a fixed potential via a load resistor.
 9. Phase-locked loopcircuit according to claim 1, wherein the voltage-controlled oscillatorcomprises integrated components being integrated in a singlesemiconductor body.